Spray-dried detergent compositions containing sodium tripolyphosphate are exceedingly well known. Sodium tripolyphosphate is a highly efficient detergency builder and its hexahydrate forms on spray-drying needle-shaped crystals which constitute an excellent porous matrix or carrier material for the organic components, notably detergent-active compounds, in the composition. Spray-dried granules containing sodium tripolyphosphate are of high particle porosity; accordingly such spray-dried powders tend to have relatively low bulk density, for example, of the order of 450 g/liter. While the porosity and structure of these granules can be highly advantageous in terms of capacity for carrying organic components and powder properties, they are necessarily associated with relatively low bulk density.
A conventional spray-dried powder built with sodium tripolyphosphate is prepared by spray-drying an aqueous slurry which will contain other salts, notably sodium sulphate, sodium carbonate, and sodium silicate, as well as the sodium tripolyphosphate. These salts, which perform various functions in the wash liquor, also may contribute to the structure of the spray-dried particles. Phosphate-built powders presently on the European market typically contain about 20 to 35% of sodium sulphate.
It has already been suggested that instead of including all the desired amount of sodium sulphate in the slurry, a part of it should be postdosed, that is to say, admixed afterwards with the spray-dried base powder. For example, one powder currently available on the French market contains 20% sodium sulphate, of which 2.5% (based on the whole powder) is included in the slurry and 17.5% postdosed. The slurry also contains 5% sodium carbonate. This powder contains a relatively high level of sodium tripolyphosphate (31%), all of which is included in the slurry. The bulk density of the final powder is about 560 g/liter.
EP 168 102A (Unilever), published on January 15, 1986, describes and claims a process for the preparation of a high bulk density detergent powder containing anionic and nonionic surfactants. A spray-dried base powder essentially free of sodium sulphate and containing some nonionic surfactant is prepared, and the remaining nonionic surfactant is in part sprayed onto the base powder and in part sorbed onto a carrier (zeolite plus sodium perborate monohydrate) and postdosed. Two powders, containing respectively 32% and 29.5% by weight of sodium tripolyphosphate, are described in the Examples; the postdosed zeolite/perborate/nonionic surfactant material in each case contains a low level (2.24% and 5.252% by weight respectively) of sodium sulphate.
JP 83/213 099A (Kao Corporation), Chemical Abstracts 100 158590k, discloses a detergent powder with good anticaking properties prepared by mixing 60 to 95 parts of spray-dried base powder with 5 to 40 parts of sodium carbonate of apparent specific gravity 0.25 to 0.70 g/ml (250 to 700 g/liter), average particle diameter 250 to 600 .mu.m and containing &gt;20% of particles &lt;125 .mu.m.
U.S. Pat. No. 3,573,930 (Philadelphia Quartz Co./Weldes et al) discloses a detergent composition prepared by a process in which a spray-dried base powder containing surfactant, builder, a low level (about 2%) of sodium silicate, and minor ingredients is first prepared, and a mixture consisting predominantly of solid hydrated sodium silicate is postdosed to the base powder.
U.S. Pat. No. 4,136,051 (Henkel KGaA/Saran et al) discloses another detergent composition prepared by a combination of spray-drying and postdosing. The postdosed material comprises sodium silicate, bleaching compounds and nonionic surfactant.
The present invention is based on the discovery that the bulk density of spray-dried powders can be increased substantially, without detriment to powder properties (flow, compressibility, resistance to caking, ease of dispensing), by a method which combines spray-drying a slurry of low salt content with postdosing a relatively high level of sodium sulphate of defined bulk density and particle size distribution, provided that the content of sodium tripolyphosphate in the base powder does not exceed a particular level.